If I have a wooden canopy over the tank, do I still need to cover it with glass/eggcrate/netting to prevent jumpers? Or just net the back part which is open? If I don't cover it with glass, will all the evaporation destroy the wood?
I think that depends on how tall the canopy is and the type of fish. Most of the time I think it would be fine, unless the fish in question can swim fast enough to jump over the canopy. I can't see most fish being able to jump more than a 6-8 inches, but I can't say I've ever seen them try to jump, so that's just a guess.
Not sure about the evaporation. I don't think it would be a problem, as there is air flow through the canopy so the humidity wouldn't be that much higher than the air in the house. I would be conserned about splashing and salt creep though.
Fish can launch a long way, I found one 5+ feet away from the tank. Glass covers are not real common because they cut down on light and have to be constantly cleaned. A screen top that covers every square inch of your tank is a must if you plan to keep wrasses and known jumpers.
The best way to keep damage to a minimum is to install a fan through your canopy for air circulation.